. A full-term pregnancy may have two opposing effects on breast cancer risk: 1) an adverse one shortly after delivery; and 2) a beneficial one in the long term. These effects could be due to group-enhancing consequences of the elevated pregnancy hormones on already initiated cells, superimposed on the long-term protective effect brought about by pregnancy-induced terminal differentiation of the susceptible mammary gland cells. The relations between birth characteristics and the risk and prognosis of postpartum maternal breast cancer have not been adequately examined. The plan is to study these relations in a database that links together the Swedish Medical Birth Registry, National Cancer Registry, and Registry of Causes of Death. Study questions include whether delivery of a large size child or a multiple birth (indicators of high levels of pregnancy hormones), a pre-term baby, a small-for-date baby or one with low Ponderal index (indicators of growth retardation), affect maternal risk of breast cancer, whether preeclampsia during pregnancy (indicator of placental dysfunction) affects postpartum breast cancer, whether breast cancers diagnosed shortly after delivery have a different prognosis compared to those diagnosed after a longer period after delivery, and whether gender of the child predicts the prognosis of breast cancers diagnosed shortly after delivery. Cases are more than 5,000 women who had one or more childbirths between 1973 and 1992 and who had a breast cancer diagnosis during the same period. To study risk factors, for each case subject, five controls will be randomly selected from among those who were in the source population during the same period, were alive at the date of the diagnosis of the index case, and had not been diagnosed with breast cancer by that date. The controls will be matched to the case on birth year and birth month. Standard statistical methods for a matched case-control study will be applied to examine birth characteristics as risk determinants of postpartum breast cancer and standard methods for survival analysis will be applied to examine birth characteristics as prognostic factors among those diagnosed with breast cancer after a childbirth.